The problem of interrupting excessive d-c currents by means of electric fuses differs in many respects significantly from the problem of interruption of excessive a-c currents by means of electric fuses. For this reason many manufacturers of fuses have two different lines of fuses, one intended for application in a-c circuits and one intended for application in d-c circuits.
Commercially available d-c fuses designed for relatively high current ratings and involving a plurality of fusible elements which are connected in parallel perform generally fairly well both on major fault currents and relatively small protracted overload currents. At lower current ratings satisfactory interruption tends to become more difficult. One of the most onerous instances is the interruption by means of a fuse having a relatively low current rating and consequently but one single fusible element of a d-c circuit having a relatively large time constant and carrying a very small overload current for a long period of time prior to blowing of the fuse. Under such conditions the back burn velocity of the fusible element tends to be so small as to make it difficult, or impossible, to force the current down to zero.
The prime object of this invention is to provide d-c power fuses for relatively small current ratings which operate satisfactorily under all conditions including major fault error conditions and small protracted overload current conditions, and also including relatively highly inductive circuits.